1) Field of the Invention
The present invention relates to a hard disc drive cover-integrated gasket, and more particularly to a hard disc drive cover-integrated gasket, which is easy to fabricate and has improved characteristics such as low outgassing and good sealability.
2) Related Art
Recent tendency toward smaller sizes and higher performance of electronic appliances is requiring smaller and thinner structural parts, consequently giving rise to deterioration of assembling efficiency in the fabrication process. To overcome such a problem, integration and combination of parts are keenly desired, together with improvements in their characteristics or performances such as outgassing, sealability, etc.
In electronic memory apparatuses, gaskets to be fitted to the hard disc drive cover particularly for preventing intrusion of water or dusts and for satisfying low-outgassing requirements have been so far fitted to the hard disc drive in the sandwich form of a single rubber sheet or a foamed polyurethane sheet between metallic covers of stainless steel, aluminum, etc. In this connection, it has been proposed to integrate a metallic cover of stainless steel, etc. with a rubber material such as fluoroelastomers, etc. by bonding with an adhesive, thereby improving the assembling efficiency (JP No. 2,517,797).
However, the proposed method is based on preparation of gasket-shaped rubber through an additional vulcanization molding step in advance, followed by bonding the rubber to a metallic cover with an adhesive, needing a prolonged series of process steps and complicating the process per se. Actually, the gasket vulcanization step needs a few minute, and the vulcanized gaskets are liable to tear off to smaller pieces or are readily susceptible to dust deposition thereon, etc., so repeated washing or screening is imperative before the assembling. Thus, a more simplified method has been keenly desired.
To overcome the foregoing problems, it has been proposed to use a gasket material comprising a polystyrene type block copolymer elastomer capable of simplifying the process step without a vulcanization step and also capable of material recycling to reduce the cost, as compared with the conventional rubber material (JP No. 2,961,068).
The proposed material is liable to become finer, softer and more sticky. Unless the material has been fixed by some means in advance, the working efficiency will be lowered in the assembling work of hard disc drives. To overcome this problem, a gasket of polystyrene type block copolymer elastomer is prepared by injection molding into the so called frame in advance, followed by inserting the framed gasket between a box and a cover of hard disc drive to effect the desired integration. After all, additional parts such as frame, etc. are required in the proposed method.
Due to the recent tendency toward higher performance of hard disc drive, gaskets to be fitted to the hard disc drive covers are inevitably exposed to elevated temperatures, e.g. 100° C. or higher. These polystyrene type thermoplastic elastomers have failed to attain satisfactory performance at such elevated temperatures.
Particularly, the gaskets to be fitted to the hard disc drive covers have strict requirements for sealability, cleanliness and heat resistance.
Three key factors that affect sealing characteristics are hardness, compression set characteristic and water permeability of the gasket. Hardness (JIS durometer type A) is preferably 60 or less. Gasket with a hardness of more than 60 fails to ensure sealing characteristic between the hard disc drive and the cover due to clearances formed therebetween by the bending backward force of the gasket, when fitted generally to assemble a hard disc drive product. Material with a poor compression set characteristic also fails to ensure sealing characteristics due to tension loss in the course of long-term use, material with a compression set of 100% or higher is impractical. Furthermore, water vapor (moisture) as well as dirt and dust etc. is included as an object to be sealed, and thus the gasket materials must have a low water permeability. Involvement of water vapor inside the hard disc drive is a cause for generating rusts.
Cleanliness-controlling factors are the following three: outgassing, components as contained and a filler which tends to drop off from the gasket. When the outgases generated from the materials deposit onto the disc, such deposition will be a cause for crushing. Components as contained in constituent materials, such as chlorine, silicon, sulfur, sulfuric acid, nitric acid, acrylate esters, etc. (including their presence in the form of ions) will also cause corrosion of hard disc drive interior. When a large amount of a filler having no affinity toward the polymers is contained in the gasket materials, the filler particles have a high possibility of dropping off from the gasket to fall into the hard disc drive interior, giving also rise to crushing.
As is concerned with the heat resistance, the hard disc drives so far used in the personal computers, etc. are directed basically to indoor use, where the service conditions are at about 0°–about 60° C., or maximum at about 80° C. even if used in continuous operation with internal heat generation. When the hard disc drives are applied to car navigators, etc., on the other hand, it is imperative that the hard disc drives mounted on cars can be immediately started even after being left standing in a cold area at low temperatures such as down to about −40° C., or can maintain their performance even after being left standing in directly sun beam-irradiated, tightly closed car interior at elevated temperatures such as up to about 100° C. In the case of the polystyrene type thermoplastic elastomer as the leading material up to now, glass transition temperature (Tg) of the hard segment is about 100° C., and thus no better heat resistance is basically obtainable above about 100° C.